Author: Chris Meyer VP of Engineering, PEBCO Inc.
|Available Space||Abrasiveness||Complete Shutoff||Single Blade Diverter (SBD)|
|Required Geometry||Temperature||Shift-On-The-Fly||Single Blade/External Access (SBDX)|
|Sealing Ability||Flowability||Speed||Single Blade Winged (SBDW)|
|Product Retention||Particle Size||Maintenance Access||Two Way Rolling Blade (TW)|
|Differential Pressure||Corrosiveness||Offset Rolling Blade (OS)|
|Flow Splitting||Multiple Materials||Basket Diverter (BD)|
|Standing Column||Bucket Diverter (DB)|
|Number of Outlets|
Space & Geometry: Diverters come in a wide variety of shapes and sizes. Basket diverters are typically the shortest style of diverter while bucket diverters and single blade diverters tend to be the tallest. The angular configuration of the diverter also has a significant impact on height. Of course, custom angular configurations are always available from PEBCO, but standard configurations utilize 90⁰ and 60⁰ included angles (also known as “Y” diverters) or 45⁰ and 30⁰ offset angles (also known and “K” diverters). Included angle diverters measure the angle between the centerline of the outlet legs and offset diverters are measured by the angle between the centerline of the offset leg and the vertical, through leg. 90⁰ included diverters are generally the shortest diverters within a product family, while 30⁰ offset diverters are the tallest.
New construction projects can usually be laid out to accommodate the necessary diverter, however existing chutework geometry within an operating plant generally dictates the configuration of a diverter that will replace one that is currently in operation. PEBCO can custom manufacture diverters that have special angles on one or more of the outlet legs to better fit existing geometry. For example, if height is a concern for a particular application but the material flows well, a 45⁰ offset diverter can be built with a 30⁰ flange on the offset leg to help in matching existing chutework while maintaining a relatively compact size.
Smaller Diverters: BD, TW, and OS Diverters
Larger Diverters: SBD, SBDW, SBDX, and DB Diverters
Sealing Ability: Although it is not always the case, the ability of a diverter to tightly seal to the closed leg is often an important concern. This can be important due to cross-contamination concerns, differential pressure, or simple leakage. Completely sealing one leg is generally not a concern in flow splitting applications as both legs are open simultaneously. Elastomeric seals often work well, but live loaded or compressed seals perform best. Temperature can play an important role in sealing since few seals provide good service above 200⁰F and 500⁰F is the maximum service temperature of silicone and Teflon products.
As seals wear, sealing ability decreases. Elastomeric seals that rub on the sides of the diverter body tend to wear relatively quickly as is seen in conventional “flop gate” designs. Elastomeric seals that make contact with moving components for only a relatively small movement exhibit substantially longer life. This type of sealing system is found in the rolling blade diverters as a cam action is used to press the blade into the seal only at the end of stroke. Rigid polymer seals can be “liveloaded” to provide a significant increase in seal life as is found in the SBDX design.
Best Sealing to Closed Leg: TW, OS, SBDX, and SBD Diverters
Good Sealing to Closed Leg: BD, DB, and SBDW Diverters (Primarily used for larger particle sizes with moderate dust content or where minor dust leakage to the closed leg is not a concern)
-Not recommended in differential pressure applications-
Product Retention: This is typically a concern when multiple products are moved through the same diverter. A diverter that is fully or mostly self-cleaning is required to minimize cross contamination. In these cases it is common for a customer to indicate that no product retention is permitted when, in fact, they mean it should be kept to a minimum. Diverters are often used to separate products into different storage vessels and the equipment bringing material to the diverter is likely to have some level of product retention inherent in its design be it a belt, screw, drag, or other type conveyor or feeder. Realistic discussions should be had with the customer regarding product retention. Assuming that a diverter with proper sealing abilities is selected and the discussion is focused around product retention within the equipment; Is a pound of Product A really relevant in a 50 ton silo of Product B?
Another concern regarding product retention is spontaneous combustion. Some products (e.g. Powder River Basin coal) tend to self-ignite if left packed and undisturbed for some period of time. This can lead to damage to down-stream conveyor belts or other equipment. Self-cleaning diverters that omit ledges where product can build up are a must in these applications.
Diverters with Minimal Product Retention: SBD, SBDX, SBDW, DB, and BD Diverters
Diverters with Some Product Retention: TW and OS Rolling Blade Diverters
Differential Pressure: While this is not a common application challenge with diverters, PEBCO has encountered instances where portions of the system are held at a negative due to bin vents or other dust control equipment. Diverters that seal well to the closed leg are required to prevent airflow from pulling dust to undesirable locations. Know where the differential pressures originate and carefully consider the implications of possible leak paths during system operation.
Best Diverters for Applications with Minor Differential Pressures: SBDX, TW, & OS Diverters
Poor Diverter Selections for Differential Pressure Applications: BD, DB, & SBDW Diverters
Flow Splitting: In some applications there is a need to split the flow of material coming through the diverter. It is important to determine what level of accuracy is required, what the split ratios are (e.g. 50/50, 70/30, variable, etc.). For both variable split ratios and high accuracy requirements, a feedback device is required downstream of the outlets. This, combined with positional control on the diverter’s actuator, can be used to allow the customer to create a closed loop feedback system that compares the desired flow rate to actual and corrects the diverter position continuously to achieve the required split. Hydraulic actuators with proportional valves are best for this application due to the stiffness of the system. Electric actuators may be used, however duty cycles must be observed. Pneumatic positioners are suitable if accuracy requirements are not overly strict (compressibility of the air leads to lower positioning accuracy).
Flow splitting diverters should have a single moving element to obtain the best possible accuracy. A properly executed
installation and well-tuned PID loop should be able to obtain +/- 5% split accuracy.
Care must be taken to insure that the incoming product stream is evenly distributed across the diverter. If the diverter is
fed from a conveyor belt, the shaft of the diverter should be oriented parallel with the direction of belt travel.
This allows the diverter to split the incoming product stream by moving the splitting device along the longest dimension of the stream. Material coming off of a belt will generally have a somewhat rectangular profile. Take for example a belt that discharges a material stream with nominal dimensions of 30” x 10”. The flow splitting diverter should articulate across the 30” dimension of the stream to achieve the best possible accuracy and minimize the impact of flow rate variations on the splitting accuracy. If the diverter were to articulate across the 10” dimension, very small positioning errors or changes in incoming flow rate could have significant impact on the flow splitting percentages.
These diverters will generally include a hardened “splitter tip” that is the primary point of contact with the incoming material. This component is typically designed to be easily replaceable.
Best Diverters for Flow Splitting Applications: SBD, SBDX, SBDW, and DB Diverters
Poor Diverter Selections for Flow Splitting: BD, TW, and OS Diverters
Standing Column Conditions: This requirement is rare in diverter applications and very few diverters offer this feature. Rolling blade diverters can offer this functionality in grain, granule, and powder applications with properly sized cylinders.
Type of Actuation: PEBCO offers all of the typical actuator styles on our diverters. Pneumatic is the most common but hydraulic, electric, and manual are available. It should be noted that many diverter designs require significant actuator forces and direct manual actuation is typically only practical up to a 12” or 14” size diverter. Beyond this, a gearbox may need to be added to keep operator force requirements reasonable. In many cases this addition may be more costly than simply going to a pneumatic actuator.
Number of Outlets: While 2-way diverters are the most common, PEBCO offers a 3-way configuration for many of our standard diverters. More outlets can be attained by stacking multiple diverters.
Abrasiveness: As with any bulk handling equipment, liner type and installation is a function of the material being handled. Very abrasive materials should be paired with very durable liner materials that are easily replaceable while moderately abrasive materials may be paired with durable but permanent liners. Some materials show little abrasive wear even on plain carbon steel or stainless steel and no liners are required for the application. The surfaces that redirect material flow within a diverter will experience the most wear due to increased contact pressures.
PEBCO offers a wide variety of liner materials including 304 and 316 stainless steel, 235, 400, and 500 Brinell plate. Chromium carbide overlay is available in both standard and blanchard-ground finishes as well as ceramic tile lining systems. Polymer liners in UHWM or Teflon are also available and discussed in more detail below. While the above mentioned liner systems are the most common, custom configurations specified by the customer are always available.
Temperature: As mentioned above, temperature plays an important role in seal selection in a diverter but temperature can also dictate the basic materials of construction of the gate. While carbon steel is serviceable to about 700⁰F, 304 stainless should be used above this temperature and up to about 1000⁰F. 316 stainless can be used for temperatures up to 1300⁰F and exotic materials such as Inconel should be considered for extremely high temperature applications. On the other end of the spectrum, stainless and exotic steels perform well in extremely cold environments but plain carbon steel becomes quite brittle below -40⁰F. Corten or A588 should be used as the basic material of construction in extremely cold environments if carbon steel is required.
Temper-hardened wear plates lose performance characteristics at temperatures in excess of about 700⁰F as annealing will reduce or eliminate the hardness inherent in the liner plate. For very high temperature, very high abrasion applications; chromium carbide wear plates are serviceable at temperatures up to 1100⁰F.
Temperature dictates component selection in both high and low temperature applications. In extreme temperature applications it is critical to identify not only the product temperature but also the ambient temperature. As an example, a process may handle a 700⁰F product and the customer desires an electric actuator.
The exterior of the equipment is exposed to ambient conditions with good air circulation. A simple thermal barrier to prevent radiant heat transfer from the body of the diverter to the electric actuator will protect the internal electronics from overheating.
However if this same need were in a steel mill over or near the arc furnace where ambient temperatures may be well over 180⁰F, sufficient protection for the motor and other electrical devices cannot be achieved to obtain acceptable results and reliable operation.
Seal selection in valves and cylinders must be compatible with the temperature the actuator is expected to operate in. Viton seals can be used in air cylinders and directional valves for high temperature applications. However, these seals result in a 90% reduction in life for pneumatic cylinder seals and may only survive a few hundred thousand cycles before failure. The cylinder’s rod seal is typically the first failure point with these seals in high-cycle applications. This makes definition of the thermal conditions in the application particularly important for valves that have a high cyclic rate.
Greases and oils must also be compatible with the operational temperatures as these fluids can become too thick or too thin to perform properly. Lubrication can also be a concern in food grade applications, regardless of temperature, as the lubricant must be FDA approved if there is a potential for direct contact with the material being handled. As discussed above, electrical devices can be particularly susceptible to high temperatures, but some limit switches require special consideration at extremely low temperatures as well.
Flowability: Because a diverter changes the direction of flow of material as it passes through, special attention should be paid to the nature of the product while flowing. A conservative approach is to size the inlet opening for an equivalent slide gate or vertical chute handling about 3-times the actual volumetric flow rate that the diverter is required to handle. This is applicable to 45⁰ offset and 90⁰ included diverters handling material with average flowability such as coal. For very free flowing materials such as dry frac sand or diverters with lesser angles of diversion (i.e. 30⁰ from vertical or less) a factor of 2.0-2.5 may be successfully utilized.
Stringy or sticky materials that tend to bridge easily require special consideration. Directional changes of 30⁰ or less are recommended. Liner selection can also help with flow problems inside a diverter. Polished stainless liners or UHWM can be used to promote flow and reduce material adhesion to internal surfaces, with UHMW being the better performer within its operating temperature range (-20⁰F to 180⁰F). Teflon has a wider operating temperature range (-350⁰F to 500⁰F) and has a lower coefficient of friction for improved material release. However, this increase in performance also comes at a significant increase in price. It should be noted that all polymer liners’ performance characteristics vary with temperature. At the low end of their operating range they tend to be brittle and do not absorb impact well. At the upper end of the range, hardness goes down reducing wear properties.
Particle Size: This property of the material should be considered when selecting any equipment for handling dry bulk solids. It is typically recommended that the size of the equipment opening be four times the largest particle size. This is particularly important in a diverter as the material flow experiences a change of direction in a diverter and is necessarily slowed as a result. This change in direction can lead to an increase in the likelihood of plugging, particularly for materials that exhibit a tendency to bridge.
Another consideration is particle size distribution. If a significant percentage of the particles are at or near the largest size the 4:1 rule must be observed. However, if the nominal product size is ¾” with <1% of particles being 3” (occasional lump), an 8” diverter may still be quite applicable depending on the required material flow rate. It is not recommended that the diverter opening be less than 2.5 X the maximum material size to allow for the instance where two maximum size particles enter the equipment at the same time.
Due to the arrangement of internal components, bucket diverters and basket diverters have a reduced flow cross section within the diverter. This reduction is typically 15%-30% in the plane of diversion while the flow opening remains full width or slightly larger in the direction parallel with the shaft. This reduction must be considered when handling larger particles or materials that tend to bridge easily.
Large particles that do not crush easily can be problematic for some diverter designs as they may prevent full closure/shifting of the moving components. Rolling blade diverters are particularly susceptible to this type of failure to fully shift.
Corrosiveness: The reactivity of materials should always be considered when selecting bulk handling equipment. PEBCO utilizes a wide variety of materials in constructing our bulk handling equipment including, but not limited to; carbon steel, stainless steel, aluminum, brass, bronze, nylon, and UHMW. Knowing the characteristics of the material being handled and insuring its compatibility with materials of construction is critical to insuring a successful installation. Multiple Materials: In addition to the issues addressed above regarding cross contamination, multiple materials through the same diverter can result in other design concerns. The equipment should be selected to accommodate the properties of all materials it will handle with regard to particle size, abrasion, temperature, flowability, and corrosion as well as address any application requirements such as cross contamination concerns.
Complete Shutoff: In some instances, it may be desirable to have a diverter that can terminate flow to both outlets. This type of application could omit the need for a slide gate or knife gate further upstream in the process. A rolling blade diverter is the best choice as the two blades can be operated independently.
Shift-On-The-Fly: Many applications require that the diverter change position while product is flowing. Most diverters are capable of this type of operation but it should be avoided with a basket diverter. As the basket changes from one position to the other, the bottom of the basket will temporarily be horizontal. This could lead to a blockage that would both plug the inlet chute and prevent the basket from completing its rotation. Rolling blade diverters are capable of changing position with material flowing, but if product size is relatively large, this operation may increase the likelihood of wedging a particle between the leading edge of the blade and the seat of the diverter. Bucket and blade type diverters are all capable of reliable shift-on-the-fly performance.
Speed: Most applications do not require the diverter to change position quickly (i.e. 2 seconds or less). A tripper diverter after a metal detector would be an example of a situation where high speed operation would be required. There is generally very little time between the moment a particle is detected and when it reaches the diversion point. High speed diverters are generally operated by a pneumatic cylinder as equivalently fast electric and hydraulic actuators are cost prohibitive. As is the case with all pneumatic systems where speed of operation is a concern, the customer should insure adequate air supply capabilities and/or consider auxiliary receivers at the equipment locations to insure sufficient volume to perform the required task in the required time.
If high speed operation is required, it is critical that this be known early in the design phase of the project. Diverters typically have heavy internal moving components rotated about a shaft. Angular momentum due to high speed operation of these components can impart significant loads at the blade/shaft interface and require more force to obtain the desired acceleration. Since these stresses and forces increase with the square of the rotational velocity, the design must account for these increases to insure reliable operation and obtain the desired speed.
Maintenance Access: The ability to access the internal features of a diverter to replace seals, liners, or other components is always an important consideration. Many diverters are designed to be maintained in place, without the need to remove the entire piece of equipment from the process line. The SBDX, BD, and DB have an access panel on one side of the diverter that allows access to the internal components. On smaller diverters, this feature can significantly reduce downtime for routine maintenance such as replacing seals, blades, buckets, or wear liners. On larger equipment the customer may find that removing the entire diverter for maintenance is the best option for any style as rigging and lifting internal components is much easier with overhead access.
Types of Diverters
Single Blade Diverter (SBD)
The single blade diverter is probably the most commonly known and used type of diverter and is often referred to as a “Flop Gate”. The basic exterior construction is quite simple and the design utilizes an interior blade, rotated by a shaft through the side of the diverter. The simplicity of this design, and others in the single blade diverter family, results in a diverter with minimal material retention properties. The blade typically includes seals that prevent material from entering the closed leg making it suitable for applications where small differential pressures exist. External bearings support the shaft which is most often driven by a lever arm and linear actuator. Rotary actuators are available but torque requirements typically make this option cost prohibitive. All diverters in the Single Blade family provide exceptional performance in flow splitting applications. This simple, rugged design has been used for almost all bulk materials in numerous applications for decades.
Single Blade Diverter/External Access (SBDX)
As the name implies, this style of diverter operates in the same manner as the SBD but allows external access to the interior of the diverter. The seal configuration in this design differs from the typical SBD seal in that the rigid polymer side seals are “live-loaded” and installed in the edges of the blade. Bolting on the access cover compresses the rubber spring behind the seals. In either diverted position the tip of the blade is pressed into an elastomer seal. An elastomer bulb seal is used to seal the gap between the bottom of the blade and the body to prevent dust migration to the closed leg. The excellent sealing capabilities of this diverter make it a good choice for many materials and applications, but it is particularly well suited for powder applications and/or minor differential pressure applications where dust leakage to the closed leg is a
Single Blade Diverter/Winged (SBDW)
The SBDW is another variation of the “Flop Gate” that utilizes a “wing” or wall on each side of the blade assembly to channel material flow to the chosen outlet leg. This approach eliminates the need for elastomer seals. A drop-in inlet throat channels material to the blade and is replaceable. Since the inlet throat protrudes into the body of the diverter, a spool piece installed in the customer’s chutework immediately above the inlet will allow for replacement of the inlet throat while the diverter remains installed in the chutework. The diverter body is horizontally flanged in the middle to allow installation of the welded blade assembly. This design lends itself well to highly abrasive applications as the design utilizes mostly modular wear components with minimal locations requiring additional bolt-in liners. Due to the large running clearances interior to the valve, some dust migration is possible to the closed leg and these diverters should not be used in differential pressure applications. However, the modular design and tendency to be used in abrasive applications makes this an outstanding diverter for flow splitting applications.
Rolling Blade Diverters (TW & OS)
Rolling blade diverters utilize curved blades and matching seal profiles to tightly close each path to the outlets. Cam action insures tight closure between the blade and elastomer seal while preventing sliding between these surfaces for much of the stroke, thus improving seal life. This cam action also seats the blade firmly into the seal upon closing to provide a very good seal for fine powders. Each blade can be operated independently to provide one leg open, both legs open, or both legs closed functionality. Alternatively, a single actuator can be used to operate the diverter with a mechanical linkage operating the second blade. In this arrangement, one leg is always open. The Rolling Blade diverters are also the only
diverter design that can effectively cut a static column of material. There are two styles of these diverters; the TW is an included angle diverter or a “Y” diverter, the OS is an offset angle diverter or a “K” diverter. These diverters are particularly well suited for powders or where small differential pressures exist but internal ledges result in minor material retention where the blades seat in the “crotch” area of the diverter.
Basket Diverter (BD)
The basket diverter uses an inlet throat to direct material to an internal 3-sided basket which sends material flow to the chosen outlet leg. The inlet throat can be welded into the body of the diverter or made as a “drop-in” style to allow for replacement. Since the inlet throat protrudes into the body of the diverter, a spool piece installed in the customer’s chutework immediately above the inlet will allow for replacement of the inlet throat while the diverter remains installed in the chutework. The basket is supported by a shaft, which is supported by external bearings. As with most diverters, the shaft is pivoted by means of a lever arm and linear actuator, although low operating forces do lend this design to rotary actuators if desired. One side of the housing incorporates a removable cover to allow basket removal in place. Short outlet legs result in small and manageable liners in this section so all internal parts with the exception of the inlet throat can be replaced without removing the equipment from the process line. The tapered inlet throat transitions to a larger basket and then to a larger yet outlet leg, thus making the Basket Diverter a self-cleaning design with minimal material retention. Much like the SBDX, large internal clearances can result in dusting to the closed leg in some applications and this diverter should not be used where differential pressure exists between legs. Basket diverters are also not a preferred choice for shift-on-the-fly applications as the basket will temporarily block product flow while it changes positions.
Bucket Diverter (DB)
PEBCO’s bucket diverter is a relatively new design that has been in use for the past five to ten years. It features a rotating, bottomless, square profile bucket that directs material flow to the selected outlet. In smaller designs, the bucket is rotated with the familiar lever arm and shaft arrangement while on larger designs (i.e. 30” and above) the shaft simply supports the weight of the bucket and the actuator attaches to the bottom of the bucket to provide rotation. Large internal clearances provide reliable operation but may allow minor dusting to the closed leg. The through design with no interior ledges minimizes product retention and yields a self-cleaning design. The “crotch” of the diverter can be made of hardened subassemblies for easy replacement and result in a diverter with excellent flow splitting characteristics. Typically the bucket is installed through the side of the diverter housing to allow replacement of this component without removing the diverter from service although standard designs of the larger sizes allow for the bucket to be removed from the top only. Much like the basket diverter and SBDW the inlet and diverter mechanism is comprised of major weldments that allow easy replacement and lend themselves well to construction of hardened wear plates. A spool piece in the customer’s chutework at the inlet of the diverter can allow this design to be fully maintained in situ.
As discussed in the maintenance section above, the ability to remove large, heavy components with no overhead access becomes a very real issue with bucket diverters due to the weight of the bucket. However, PEBCO’s history and ability to design specialized equipment that meets the customer’s needs have resulted in custom designs that assist with these rigging concerns and allow even very large buckets to be removed through the side of the diverter.
Another customer wanted a very large SBDW that was to be installed in an existing indoor application where the only access was through a maintenance elevator. The entire diverter was created as a bolt together design that would fit through the available clearances to get to the installation location allowing in-place assembly.
These are just two examples of how PEBCO can design equipment that meets the specific needs of our customers. While these types of custom modifications can be costly and result in longer than normal lead times, they can provide long term maintenance cost savings, safety improvements, and installation benefits for customers with very specific needs.